It is known that when acoustic waves propagate in piezoelectric media, they co-propagate with electromagnetic energy. The electric fields travel at the velocity of the acoustic wave rather than at the speed of light. These fields decay evanescently into the surrounding air or vacuum that bounds the piezoelectric medium. Because the electromagnetic wave travels at the acoustic velocity, the wavelength associated with the evanescent decay is the acoustic wavelength and not the radio-frequency electromagnetic wavelength, which may be longer by four or more orders of magnitude.
It has also long been known that the evanescent electromagnetic fields can be used to couple surface acoustic waves (SAWs) across an air gap between propagation media. This principle was demonstrated in 1969, as reported in W. L. Bond et al., “Acoustic Surface Wave Coupling Across an Air Gap,” Appl. Phys. Lett. 14, 122 (1969) (hereinafter, “Bond et al.”). Bond et al. reported evanescent coupling across an air gap between two lithium niobate SAW delay lines.
Evanescent coupling between guided acoustic waves can potentially be exploited for radio-frequency signal processing. For example, it could allow for frequency-dependent or frequency-selective processing by resonant structures that are of tractable dimensions because they are scaled to the acoustic wavelength instead of the electromagnetic wavelength.
However, the results reported by Bond et al. failed to achieve widespread practical application due in part to the difficulty of setting a precise air gap between two very thick lithium niobate substrates for very long distances. Although coupling of signals between two separate chips was achievable, there was no simple path to the successful integration of two coupled acoustic waveguides on the same chip, as would be necessary in a practical device for radio-frequency signal processing.